Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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~l ctricallv Conductive T _ _e r _loor Cover ng
The invention relates to an electrically conductive table or floor
covering which ;ncludes an under layer of conductive rubber or plastic which
1s appropriately shaped and electrically connected to ground by means of a
metallic conductor, and an upper layer having a greater resistance than the
under layer.
A floor covering of this type is described, for example, in DE-OS 28 24
739, the upper layer of which is continuously formed snd covers the under
layer over its entire surface. Electrical charge accumulated at the surface
of the upper layer MUSt be shunted through the material of the upper layer to
the under layer, which is extremely difficult to achieve in a reliable manner.
Antistatic floor coverings are needed, for example, in operating rooms and
rooms in which electrical data processing systems or explosive materials are
stor0d. The discharge capacity is normally defined by the resistance
thereto. According to the area of application, this has a varying magnitude
and should, in accordance with regulations, be between 2 x 10 and 10
Ohms for operating rooms; in rooms in which electrical data processing systems
are stored, less than 10 Ohms; and in storerooms for explosive materials,
less than 10 Ohms. The equipping of these various rooms thus necessitates
floor coverings of varying construction, which is clearly unsatisfactory from
the viewpoint of production and storage costs.
The adjustment of the discharge resistance is achieved in the case of
floor coverings according to DE-OS 28 24 739 by the incorporation of selected
amounts of chemical substances effective as antistatic agents in the material
used to produce the upper or lower layer. Alkyl sulfates, alkyl sulfonates,
quaternary nitrogen bases, ethoxylated fatty amines, alkyloamines, phosphoric
acid esters, polyglycol esters and soots are mentioned in this publication as
being suitable antistatic agents. However, these have the disadvantage of
being easily washed out by water used in washing the floor covering or that of
darkening the floor covering, which i9 equally unsatiseactory. Nevertheless,
the achievement of a selected discharge resistance of the entire floor
covering only by the application of a suitably modified material mixture is
extremely difficult in that the discharge resistance is, among other things,
ultimately determined by the thickness of the individual layers as well as by
the hygroscopicity of the material used, and therefore by the dimensions
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which, during the subsequent use of the floor covering, are subject to
considerable change.
The present invention addresses the problem of providing a standardized,
electrically conductive table or floor covering, which is impervious to the
effects of water and abrasive wear and tear and meets the safety stanclards
applicable to rooms wherein antistatic coverings are required. It should also
be possible to provide the table or floor covering with a light-coloured or
colourful upper surEace, if desired.
Our Canadian patent application No. 438,649, filed October 7, 1983,
describes a process for joining together the individual layers of an
electrically conductive floor covering. Such process, as described below, may
be applied to the production of table and floor coverings according to the
present invention. In this process, the upper and lower layers are pressed
together and the upper layer has regularly or randomly distributed
perforations therethrough, these being of a minimum diameter of 1 mm and so
disposed that the smallest distance between them is no less than 2 mm and the
largest no more than 40 mm. The pressing process is such that the material of
the under layer fills the perforations made in the upper layer right up to the
surface of the upper layer.
This process is particularly suited to the present invention because the
upper and lower layers and the isolation layer can be compressed and bonded
together simultaneously.
The upper layer is produced from any material that, when pressed together
with the under layer, will display a higher flow viscosity and which, because
of its type, is suitable for forming a solid bond with the material that makes
up the under layer. Such a coordination of the viscosities of the upper layer
and the under layer is achieved by the choice of correspondingly different
materials for both layers. The under layer may for instance consist of a
rubber-elastic material, the upper layer of a thermoplastic material.
Polyesters and polyamides are particularly suitable, wh0reas sheets such as
woven fabrics, knits or bonded fabrics can be used to produce the upper
layer. Insofar as the thermal conditions of the pressing process bring about
a softening effect in the upper layer, it is simply necessary to ensure that
the flow viscosity of the upper layer material is greater than the flow
viscosity of the material in the uncler layer that is softened at the same
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time. This will lead to sood contour sharpness between the variously coloured
areas constituted by the projections of the under layer seen in the perforated
regions of the upper layer. The flow viscosity that is referred to is that
which is designated "Mooney viscosity" in the case of rubber-elastic materials
It is necessary to ensure a sufficiently stable bond between the upper and
under layers and this can be improved if necessary by incorporating a layer of
adhesive agent, for example, a polyurethane adhesive, a polychloroprene
adhesive, a resorcinol-formaldehyde-latex adhesive, or a chlorine-rubber
solution. In the case of open-weave layers - for example, a net or a fleece -
a layer of this kind will simultaneously hinder the penetration of the poresof the upper layer by the rubber-~lastic material that is softened during the
vulcanization process.
Where the under and the upper layers are of the same eeneral type of
material - for e~ample, a rubber-elastic material - it is possible to achieve
an increase in the flow viscosity of the material in the upper layer during
the pressing process by mixing iD a relatively large amount of a mineral
filler. In the same way, the flow viscosity of the material in the lower
layer can be reduced by using or mixing in a copolymer material of especially
low viscosity, or by usin~ another type of vulcanization material. It is also
possible to produce the upper and the under layers of identical rubber-elastic
materials ~the conductive additives and/or amounts thereof will be different~,
if the upper layer is vulcanized and synchronous softening during pressing
together of the upper layer with the lower layer is hindered thereby.
Because of the electrically isolating layer forming the bottom of the
table or floor covering according to the invention, the discharge resistance
can be adjusted to a precise value entirely independently of the resistances
of the table or floor. This is accomplished by means of a variable resistance
element arranged in the metal].ic conductor, which connects the under layer
with the ground wire. Subsequent adjustments can be carried out by unskilled
help, if necessary, and can also be achieved automatically if the variQble
resistance element is connected to Q self-activQting control device - such
systems being commercially available.
The advantages realized by the table or floor covering according to this
invention are mostly due to the fact that the electrical resistance can be
adjusted to any desired value by activating the variable resistance element
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with raspect to the requirements of each individual case. It is thereby
sufficient to have a standardized embodiment of the table or floor covering of
the invention for most applications and the storage heretofore required is
thereby greatly reduced and the production much simplified. A further
sdvantage is due to the fact that the table or floor covering of the invention
can be designed to have light colours, which is very desirable if attractively
equipped rooms are required.
The invention will now be described further by reference to the following
example.
EXAMPLE _
A rubber mixture A (see below) is homogenised in an internal mixer. The
mixture is then passed to a calender and rolled out to a sheet 0.5 to 1.0 mm
thick. The sheet is rolled and subsequently passed to a strip calender in
which it is prevulcanized at a surface temperature of 180 deg. C at a
throughput rate of 80 m/h for a period of approximately 3 minutes.
The components listed as mixture I below are introduced to the mixer and
after homogenisation has been completed, the mixture is rolled out to form a
sheet 1.5 to 2.0 mm thick.
A portion of the sheet of composition A is provided with perforations
which penetrate it completely, in a roll punch. These perforations are
cylindrical and are 1.8 mm in diameter and are arranged 20 mm apart in the
transverse and longitudinal directions. The perforations as a whole form a
mosaic that is reminiscent of a tile pattern. In the sense of the present
invention, this sheet constitutes the upper layer. It is inherently strong to
the point that it can be handled without any danger of damage or tearing the
pattern formed by the perforations.
The sheet produced from the mixture according to Mixture I is then taken
and covered on its upper side with the perforated upper layer and on its
underside with the unperforated remaining portion of the sheet of mixture A.
The three-layered laminate so obtained is then transferred to a heated
calender. The upper layer with the blanks is directed to the polished
calender roller; at a surface temperature of 180 C, this has a running speed
of 36 m/h, which corresponds to a residence time of 6 minutes for the
triple-layered construction. During this period, the three layers are
vulcanized and inseparably joined together. The surface i5 characterized by a
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full, continuous vulcani~ation skin whose colour pattern is broken only by the
material of the under l~yer pressed into the blanks.
A similar result may be obtained i instead of the polished calender
roller, a roller is used having a waffle or leather-like surface. Other
suitable pressing tools may also be used.
The under layer is then connected to a metallic conductor having a
variable resistance, whereby the discharge resistance can be varied from
5 x 10 to 5 x 10 Ohms.
MIXTURE A MIXTURE I
SBR-rubber 15 10ll . 8 %
High styrol resin (6570 block-styrol~ 2.4 %O 5.6 %
Kaolin 60.0 % qO
Carbon black 61.5 %
Chalk 8.5 qO 14.9 %
Softener 1.7 % 3.35 qO
TiO2 2.5 7O
Lithopone 6.7 %
ZnO 0.7 % 0.7 %
Stearic acid 0.35 qO 0.35 %
Triethanolamine 0.35 %
Paraffin 0.35 % 0.35 7O
Anti-aging agent 0.2 % 0.2 %
2-Mercaptobenzothiazol0.2 % 0.2 qO
Dibenzothiazyldisulfide0.2 qO 0.2 %
Sulfur 0.85 qO 0.85 %
100.00 7O 100.00 %
Figures given in percent, relative to the total weight of the mi~ture.
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